The invention relates to a device of the generic type defined by the preamble to claim 1.
Such a device can be embodied as a stand-alone diagnostic device, or as a component of an implantable cardiac pacemaker with an anti-tachycardial mode of operation, a defibrillator, cardioverter, or combination device having the function of both of the above devices.
The therapy of tachycardial disturbances in heart rhythm is one of the most important fields in cardiology and is especially a primary field of application of electrostimulation of the heart.
In view of the many ways these problems can be manifested and their many causes, extensive attempts have been made in recent years to improve the preconditions for therapeutic success by more-precise classification of tachycardial arrhythmias and more-exact association with typical defects in the cardiac stimulus conduction system. Because they are comparatively widespread and need treatment, on the one hand, and on the other because of the good chances of successfully treating them using special pacemaker pulse trains, the problem of reliably distinguishing pathological ventricular tachycardias from other forms of arrhythmia--such as physiological tachycardias, ventricular fibrillation or supra ventricular tachycardias--and under differentiated conditions as well--has come to be recognized as especially important.
European Patent Application EP 0 094 758 A2 has proposed that a criterion for the presence of a pathological tachycardia be obtained by comparing a current time interval between successive heartbeats with a previously determined mean value on the one hand and with a predetermined (fixed) value on the other. Thus some progress was gained compared with the earliest diagnostic methods, which were based solely on the evaluation of the current heart rate and could not distinguish physiological tachyarrhythmias from pathological ones. For exact classification of the various types of tachycardia, however, the confidence level of this method is insufficient.
In a further development of this method, it is proposed in German Patent Disclosure DE 44 39 256 A1 that the relative distribution of heartbeat intervals, which can be considered fibrillation intervals or tachycardia intervals, within predetermined ranges of time be used in order to classify the prevailing type of arrhythmia. This is intended to take into account the fact that the internal lengths often overlap considerably in practice between ventricular tachycardia on the one hand and fibrillation on the other. However, since in the final analysis classification is done solely on the basis of the intervals between ventricular actions (called the "RR intervals"), the problem cannot be attacked at its roots by proceeding in this way.
Later, various proposals have been made--for instance in European Patent Disclosure EP 0 302 577 A2--that the signal form of the EKG (taken conventionally or intracardially) be used as the distinguishing and even predicting criterion. The EKG signal form, however, does not adequately reliably reflect the occurrence of ventricular tachycardia, and the high signal resolution needed to achieve a fair confidence level entails very high expense for measurement and processing, which is hardly feasible in an implantable pacemaker.
In the proposal for tachycardia classification made in European Patent Disclosure EP 0 580 128 A1, the expense is increased still further by the additional provision of an activity sensor and of processing means for the sensor signals.
Quite a number of other approaches--which will not be discussed in detail here--make use of analysis of the spatial propagation or correlation of depolarization in the heart tissue. This requires the implantation of many electrodes for signal detection--and for this reason if no other has very little likelihood of realization.
In U.S. Pat. No. 4,860,749, a method for distinguishing a ventricular tachycardia from a sino- or other kind of supraventricular tachycardia is described, in which the atrial and ventricular heart rate (the respective reciprocals of which are also called the "PP interval" and "RR interval" below) and the AV interval (hereinafter sometimes also called the "PR interval") are measured. If the RR interval is within a predetermined range and is shorter than the PP interval, then the status is readily classified as ventricular tachycardia. If the atrial and ventricular rates are approximately the same as a consequence of 1:1 AV conduction or retrograde conduction, then the measured AV interval is subjected to a comparison with a predetermined value ("sino-AV interval"), and from the result of the comparison the classification criterion is obtained.
Proceeding in this way is also employed in the version proposed in U.S. Pat. No. 5,107,850, but in which further processing follows: For the case where the measured AV interval is longer than the predetermined one, the "regularity" (a measure of the constancy over time) of the measured AV intervals and the ventricular rate (whose reciprocal will hereinafter also be called an RR interval) are ascertained and evaluated, but this requires still a second predetermined value for the AV interval. On the basis of -additional measurement of the atrial rate and the determination of the regularity and of the atrial and ventricular rates, an atrial tachyarrhythmia can be classified.
The procedure is also quite similar in U.S. Pat. No. 5,383,910.
In these last-named methods, the specification of one or even two different AV values, which is typically done the first time the device is programmed, puts additional adaptation parameters into play, which can limit the efficiency of the algorithm employed considerably if they are not aptly chosen at the outset or if they become invalid without being noticed--for instance as a consequence of physiological changes in the patient.
U.S. Pat. No. 5,325,856 proposes a method for distinguishing between ventricular and supraventricular tachycardias, which is based on a comparison of the divergence over time in the PR and RR values, with two predetermined threshold values at the onset of the tachycardia. Once again, largely arbitrarily selected parameters play a decisive role in classifying the arrhythmia statuses, which involves the risk of mistaken associations.
U.S. Pat. No. 5,327,900 describes a method for distinguishing between pathological and physiological tachycardias at comparable atrial and ventricular rates, which is based on associating the measured AV interval with a predetermined AV time slot that has been determined from the AV interval during normal sinus rhythm. This algorithm is comparatively simple; however, so far there is no evidence of adequate efficiency of discrimination on its part.
Another method, in which a great number of additional criteria (on the RR interval and on the ratio between the PP and RR intervals) is employed to ascertain the treatability of an ascertained tachyarrhythmia in the event of virtual agreement between the atrial and ventricular rates, is describes in U.S. Pat. No. 5,379,776.
From J. M. Jenkins et al, "A Single Atrial Extra Stimulus Can Distinguish Sinus Tachycardia from 1:1 Paroxysmal Tachycardia", PACE Vol. 9/II (1986), and J. M. Jenkins and S. Caswell, "Detection Algorithms in Implantable Cardioverter Defibrillators", Proc. IEEE 84, No. 3, 428 (1996), a method is known in which by early atrial stimulation and by observation of the ventricular reaction, a distinction can be made between sinus tachycardia on the one hand and other types of tachycardia on the other, such as ventricular tachycardia with retrograde 1:1 conduction. However, the distinction possible with this method is of little clinical value; for the operation of a cardioverter defibrillator, especially, a distinction between ventricular tachycardia (VT) that requires treatment and supraventricular tachycardia (SVT) that does not require treatment must be made.
In EP 0 597 459 A2, a method based on this is described, in which if the RR and PP intervals match, first a comparison of the length of the AV or PR interval with a predetermined base value is made and finally--if this does not lead to a conclusive result--a test stimulation pulse train with a predetermined elevated pulse rate is emitted and the classification is done on the basis of the stimulated heart response, in particular changes over time in the RR intervals and in the magnitude of the AV interval. This procedure involves the danger of inducing or accelerating a ventricular tachycardia.
In EP 0 647 150 A1, a device for distinguishing between ventricular and supraventricular tachycardias is described, in which in synchronization with the R wave, the fat cushion adhering to the sino-atrial and/or AV node is stimulated, and the tachycardia is classified as a function of the ascertained change in heart rhythm. This requires that additional electrodes be implanted along with a conventional stimulation electrode line and therefore increases the expense and effort of the operation and the risks for the patient.